Several methods for milling boron nitride, in particular, hexagonal boron nitride (“h-BN”) are known in the art. One conventional process for milling h-BN is disclosed in Hagio et al., J. Am. Cer. Soc. 72:1482-84 (1989) (“Hagio”). According to Hagio, a virgin h-BN powder (characterized by a particle size of about 10 μm, a surface area of about 5 m2/g, and a thickness of about 100 nm) is milled by grinding with tungsten carbide mortar (WC) in air. The apparent purpose of Hagio's milling operation is to increase the surface area of the h-BN powder, thereby increasing its reactivity. When milled in this manner for 24 hours, the resultant h-BN powder has a lower particle diameter (2 μm), a higher surface area (54 m2/g), and is slightly thinner (71 nm). The data reported by Hagio suggests that the final geometry of the milled powder is not dependent upon the starting powder purity. Although Hagio reports a reduction in the platelet thickness, Hagio's milling operation primarily results in BN particle fracture, thereby reducing the particle diameter, resulting in an increased surface area.
In U.S. Pat. No. 5,063,184 to Tsuyoshi et al. (“Tsuyoshi”), it is reported that high surface area, highly reactive h-BN powders are useful in providing high density, pressureless sintered h-BN components. In each example in Tsuyoshi, the virgin h-BN is milled in either air or nitrogen.
The present invention is directed towards providing an improved milling method for producing h-BN powders.